In these studies the psychotropic effects of the prototypical stimulant, d-amphetamine, were compared to those of another stimulant and to the effects of a sedative and an opioid. These comparisons were conducted in subjects with histories of stimulant and either opioid or sedative abuse, and were carried out using two different types of procedures simultaneously. The first procedure was a drug discrimination task. In the drug discrimination procedure subjects were trained to respond on one lever following administration of the training dose of d-amphetamine and on another lever in the absence of d-amphetamine. Correct responding was reinforced by money. The second procedure was a traditional abuse liability assessment procedure that utilized physiologic and self-report measures. To date two studies have been conducted. In the first the effects of amphetamine and hydromorphone were compared. In the second the effects of amphetamine, methylphenidate, and diazepam were compared. In both studies amphetamine dose-relatedly occasioned d-amphetamine appropriate responding. Methylphenidate, also, dose-relatedly occasioned d-amphetamine appropriate responding. In contrast neither diazepam nor hydromorphone occasioned d-amphetamine appropriate responding. The subjective effects of d-amphetamine and methylphenidate were similar and covaried with their discriminative effects, while the subjective effects of diazepam were clearly different. In contrast, the only self-report measure that distinguished hydromorphone from d-amphetamine were drug identifications. Thus these studies show that drug discrimination procedures can be drug-class specific in humans, and that while these discriminative effects can covary with the subjective effects of the drug, the discriminative effects of amphetamine under these conditions appear to be controlled in a manner most similar to the identification of a drug.